Familial hypertrophic cardiomyopathy (FHC) is an autosomal dominant disease originating from mutations in genes that encode for the major contractile proteins of the heart, including the ventricular myosin regulatory (RLC) and essential (ELC) light chains. FHC results in ventricular and septal hypertrophy, myofibrillar disarray and is the leading cause of sudden cardiac death in young individuals. This research is aimed at elucidating the molecular mechanisms involved in triggering of FHC at the level of a single myosin cross-bridge. We propose to test the hypothesis that FHC is caused by inefficient utilization of ATP by cardiac muscle due to alteration of myosin cross-bridge kinetics in transgenic mouse hearts expressing disease-causing mutations in myosin RLC and ELC. We will examine this hypothesis at the single molecule level in papillary muscle fibers from transgenic mouse hearts which carry disease-causing mutations in the regulatory and/or essential light chains of myosin. We strongly believe that the unambiguous determination of myosin cross-bridge kinetics must be carried out at the level of a single cross-bridge and the results compared to cross-bridge mechanics derived from measurements on skinned and intact muscle fibers. The advantage of the single molecule approach is its ability to avoid averaging over ensembles of molecules with different kinetics such as a mixture of WT and FHC molecules, and the ability to unambiguously determine the kinetics of healthy and diseased muscle. Since human patients are heterozygous for FHC mutations and their thick filaments contain interspersed WT and HCM mutant heads it is extremely important to correlate the single molecule information with the phenotype of FHC assessed at the muscle fiber level. Specifically we ask whether the durations (Aim 1A) and lifetimes (Aim 1B) of detached and strongly-bound states are the same in a single cross-bridge from FHC hearts and in healthy transgenic controls. The information derived using this single molecule technology will be paralleled with functional studies of force development, ATPase on skinned papillary muscle fibers as well as force and calcium transients on intact muscle fibers from transgenic mice (Aim 2A). The ultimate objective is to link the single molecule derived data with the cellular findings to fully understand the mechanism of action of the individual RLC and ELC mutations causing FHC (Aim 2B). The fundamental question that is being addressed is why and how these individual mutations in RLC and/or ELC cause variable disease phenotypes in humans ranging from relatively mild to malignant clinical FHC phenotypes. We believe that integration of molecular biology approaches with high resolution optics and nano-fluorescence spectroscopy will enable us to successfully answer important questions regarding the molecular basis of FHC-mediated pathology in the heart and the role of RLC and ELC in cardiac muscle contraction.